Wave transmission circuits



May 27, 194T.

w. VAN B. ROBERTS WAVE TRANSMISSION CIRCUITS 2 Sheets-Sh et 1 v OriginalFiled Sept. 50, l936 HIGH P46 PM HIGH l/l/ BAND PASS INVENfOR warm m a.ROBERTS FILTER Ai'ToRNE May 27; 1941 w. VAN B. ROBERTS 2,243,440

WAVE TRANSMISSION CIRCUITS Original Filed se t. 50,, 1936 2 Sheeis-SheetHuh HII

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INVENTOR nmrm mu meme-22's ATTORNEY STATI. OFFICE WAVE TRAN SMISSIONCIRCUITS Walter van B. Roberts, Princeton, N. J., assignor to RadioCorporation of America, a corporation of Delaware Griginal applicationSeptember 30, 1936, Serial No. 103,230. Divided and this applicationSeptember 29, 1937, Serial N 166,248

2 Claims. (Cl. 178-44) My present invention relates to electrical waveFig. 4 shows a band pass filter employing simitransmission networks, andmore particularly to lar reactance networks and a pair of transconhighfrequency networks of the filter type emductances, ploying electroniccoupling devices. This ap pli- Fig. 5 shows an element of a section of aband cation is a division of my co-pending application 5 pass filteradapted to have increased uniformity Serial No. 103,230, filed Sept. 30,1936, now Patent of iterative admittance over a large portion of No.2,184,400, December 26, 1939. the transmitting band, and a high degreeof The main object of this invention is to provide relative attenuationat a pair of frequencies on a wave filter utilizing electronic devicesupon either side of the band, and whose transconductances and/ortransimped- Fig. 6 shows a receiving system embodying ances the cut-01ffrequencies and the attenuation band pass filters constructed inaccordance with of the filter depends. the invention.

Such a filter possesses the important advantage Referring now to Fig. 1,there is shown a four over filters of known types that the behavior ofterminal network terminated by an impedance is. the filter may becontrolled by biasing potentials The shunt elements Z1 and Z2 are purereactance for the electronic devices. By suitably adjusting networks ofany nature. The dotted rectangle Y the biasing potentials of theelectronic devices represents transconductance devices carrying out notonly can the transmitting range of the filter the following function: Apositive potential e1 at be a te ed at Will, but a so t e gain, or, ifdesired, the left side of Y causes current e1 1112 to fiow into theattenuation of the filter may simultaneously the rectangle from theright without any correand independently e adjusted Since the spendingfiowoutward from the left. At the same ation of biasing potentialsrequires substantially time a positive potential 62,01; th right id of Yno power, the filter characteristics may be auto causes current e2 y21to flow out of the rectangle matically controlled in accordance, forexample, with the strength of the signals traversing the 25 filter.While these filters differ from ordinary or introd cm into a circuit aVolta e rofilters in that there is no reactive coupling bevlces f u g gp portional to current in another circuit, without tween elements, theiruse is subJect to the same r considerations and will be readilyunderstood by reactwn of any Sort on the first clrcult' In the from theleft. In a similar manner rectangle Z represents transimpedance devices;that is, de-

oneiskmed in the art of wave filters. diagram where am is the voltageinduced in Z2 The novel features which I believe to be charby unitCurrent in Z1, and 221 is the Voltage acteristic of my invention are setforth in parduced m Z1 by 111111? current m the arrows Wlth' ticularityin the appended claims; the invention in the rectangle indicate heirection Of the itself, however, as to both its organization and volta eindu ed when the dlrectlons of the curmethod of operation will best beunderstood by rents are as shown by the solid headed arrows.

reference to the following description taken in The iterative impedanceis of the four terminal connection with the drawings in which I havestructure is found as in the case of. an ordinary indicateddiagrammatically several circuit OI- 3 1' ection; by olving for theinput impedganizations Whereby my invention may be Carried ance e1/i1with is considered as an arbitrary terinto effect. 40

minating impedance, and then equating this input impedance to k. Thisequation defines lo. Having found It, the current or voltage ratio pertransimpedances which are dissimilar section of filter terminated y itsiterati.Ve im- Fig. 2 shows a high pass filter section utilizing l5 fi fk 15 P y the 62/61 similar reactance elements-and a pair of screen 22/21and Subt1tutmg m the expresslon for grid tubes as transconductanceseither of these ratios the value of already found.

Fig 3 Shows a band pass filt r Section utilizing The carrying out of thecalculation above outimilar reaetance tw rk and pair of screen lined issimple but tedious, and it will be sufiicient grid tubes so connected asto perform the funchere to note that as a result there is secured thetions of transimpedances, I following:

In the drawings: Fig. 1 shows schematically a filter section utilizingreactance elements, transconductances and Now since both Z1 and Z2 arepure reactances, the first term in the denominator of Equation 2 is pureimaginary. On the other hand, all the quantities under the radical signof Equation2 which is the current, or voltage, ratio per section for therange of frequencies specified by requiring the quantity under theradical of Equation 2 to be positive.

In the case of ordinary filters composed of inductances and capacitiesonly, the theoretical filtering action that occurs per section of afilter having an infinite number of sections may be fairly clearlyapproximated in a filter having only a few sections provided the latteris terminated by a resistance which is preferably taken equal to thevalue of the iterative impedance of the last filter section evaluated atthe mid-frequency of the band in the case of a band filter, or at a verylow frequency in the case of a low pass filter, or at a very highfrequency in the case of a high pass filter. It will be understood thatthe same considerations apply in the case of the present invention, andthat preferably when the iterative impedance of the filter section(given by Equation 1) is changed the terminating resistance will becorrespondingly changed.

To provide a true filter action the ratio given by Equation 3 should beindependent of frequencies over a range of frequencies. If 1 12 and 1/21and em and 221 are each independent of frequency, the ratio is alsoindependent if yizzzi yzizm, or if y12=y21=0 or if z12=z21=0. The firstof these conditions leads to structures having two transmission bands,or, as a special case, a structure amplifying all frequencies uniformly.The second and third conditions, however, lead to structures muchsimpler, and of more practical importance, some of which will bedescribed more in detail.

Fig. 2 shows a high-pass structure where z1z=z21=0 and Z1=Z2=1/y. Inthis case the current ratio per section is simply:

'yiz y PW/21 ti /121121 which is constant for all frequencies from infinity (where 1:0) down to the frequency at which lyl l/ 21122121 andthroughout the range of frequencies so defined has an absolute valueadjusted to a point of maximum reversed plate 7 current so that theinternal plate resistance is.

extremely high, while the transconductance between control grid andplate current is negative.

Of course, in any of the filters of the invention any suitable negativetransconductance device may be used. In fact, an ordinary tube may beused if means are provided for exciting its grid with voltageproportional to e2 of Fig. l, but opposite in phase from e2. However, asthe particular devices that may be used in the electronic portions ofthe filter are not part of the present invention, they will be indicatedmerely symbolically by a circle including a control element and acurrent electrode. Whatever kind of device is used, means for supplyingoperating and control voltages, and for blocking off undesired directcurrent potentials where necessary, will be understood to be supplied inany of the well known ways.

Since the cut-off frequency of the high pass filter of Fig. 2 isdetermined by the value of while the gain per section is this filter maybe controlled both as to gain and cut-off by suitable variation of 11 12and 11 21. If capacitive admittances are substituted for the inductiveadmittances y of Fig. 2, the filter becomes a low pass filter, and mayadvantageously be used in the audio frequency system of a radio receiverwhere the audio frequency gain and cut-off frequency, or both, aredesired to be controlled automatically.

For example, in receiving weak signals accompanied by considerable noiseit is desirable to lower the cut-off frequency of the audio system. Thismay be done automatically by controlling either 11 12 or 1 21, or both,by a voltage derived from rectification of the incoming carrier. If both3112 and 11121 are varied by similar factors, the gain will remainunaltered. However, if a control voltage developed from rectification ofthe audio output of the detector is applied to superpose an oppositevariation of 3112 and 1/21, the band width will not be affected by thissuperposed variation but the gain will be altered in accordance with theaverage strength of audio signals. This control may be used either forcompressing or expanding the range of audio intensities.

Fig. 3,shows a low impedance band pass filter where Z1=Z2=Z, y1z=y21=0,and 212 and 221 are independent of frequency. Both the tubes I and 2 areordinary screen grid tubes. The voltage on the grid of tube I isdirectly proportional to input current and inversely proportional tofrequency, while the induced voltage in the output circuit is directlyproportional to plate current of tube I and to frequency. Likewise, thevoltage on the grid of tube 2 is proportional to output current andinversely to frequency while the voltage induced in the input circuit bythe plate current of tube 2 is proportional to its current and tofrequency. Hence, the induced voltage in either circuit is proportionalto current in the other circuit, and independent of frequency. As apractical matter, to avoid increasing reactive coupling throughunavoidable tube capacity, it is advisable to connect the grid of thetube 2 acting as 221 across only a fraction of the total capacityreactance of the output branch as shown in Fig. 3. In this. case, fromEquations 1, 2 and and the current ratio per section is over the band offrequencies where and the current, or voltage ratio per section is overthe band of frequencies wherein Fig. shows a particular structure thatmay be substituted for the reactance networks of Fig. 4 to provide amore uniform value of iterative impedance over the transmitted bandtogether with high relative attenuation of frequencies just outside ofthe band. The high attenuation is obtained by arranging the structure tobe series resonant at frequencies just outside the band on either sidethereof. The uniform value of 1/10 is obtained by making the admittanceof the structure low relative to over most of the band, the approach ofresonance at the outlying frequencies, however, insuring that theadmittance rises rapidly near cut-ofi to the value vymyzi which it musthave for cut-off. In each of Figs. 2, 3 and 4 there is shown theterminating resistance K across the output terminals of the filter; themanner of choosing the proper termination has been discussed previously.

In Fig. 6 there is shown a superheterodyne type of radio receiver whichemploys an I. F. amplifier consisting of a two section band pass filter;the filter sections being essentially those of Fig. 4. The amplifier.tube II! has its input electrodes coupled across the resonant inputcircuit I I, the latter being tuned to the operating I. F. which may bechosen from a range of 75 to 450 k. c. The I. F. energy source may beany desired type of converter, or first detector, network; those skilledin the art are fully aware of the superheterodyne type of constructionwherein the I. F. amplifier is preceded by one, or more, stages oftunable radio frequency amplification and a first detector. The resonantcircuit I2, tuned to the I. F. is connected between the outputelectrodes of the amplifier Ill. The negative tramsconductance elementof the first filter section comprises the .tube I3, whose anode isconnected to the high alternating potential side of input circuit II.The anode of tube I3 is maintained at the desired positive voltage byconnecting the low alternating side of input circuit II to the source ofpositive voltage B (not shown). A direct current blocking condenser I4is connected between the grid of tube I0 and the anode connection to 7circuit II. The cathodes of tubes Ill and I3 are grounded, and initialnegative grid biasing sources I5 and I6 furnish the grid biases fortubes I0 and I3 respectively. I

The input electrodes of tube I3 are coupled to the circuit I 2 by meansof the reversing transformer II; the secondary winding II of the latterbein connected in series, between the grid of tube I3 and the negativeterminal of bias source I6, with a pulsating current filter resistor I8.The section comprising circuits II, I2 and tubes I0, I3 provides a highimpedance band pass filter. The grid of tube I3 is excited with avoltage degrees out of phase with the output voltage developed acrosscircuit I2 by virtue of the reversing transformer I1. Hence, there issimulated the eifect of negative transconductance between the input andoutput circuits I I, I2.

The following band pass filter section includes amplifier I9 whose inputelectrodes are connected across the circuit I2. The negativetransconductance element for this second section is provided by tube 20having its grid connected to the secondary winding 22' of reversingtransformer 22. The filter resistor I8 is connected in series betweenthe winding 22 and the negative terminal of bias source I6. The anode oftube 20 is connected to the positiv voltage source B through the coil ofcircuit I2. The output circuit 2| is tuned to the operating I. F., andit is connected between the output electrodes of amplifier I9. Thesources I5 and I6 provide the normal negative biases for the grids oftubes I9 and 20 respectively; the cathode of the latter tubes beinggrounded. The reactances of circuit I2 are one half as large as thereactances of the end circuits II and 2I of the filter.

The terminatin resistance of the filter comprises the diode rectifiers24 and 25, the load resistors 24' and 25' thereof, and the audioutilization network. The diode 24 has its anode connected to the highalternating potential side of circuit 2I through the condenser 26. Thediode cathode is at ground potential. The load resistor 24 is connectedbetween the electrodes of diode 24, and the audio frequency component ofrectified I. F. current is impressed upon the grid of audio tube 311through condenser 3I connected to the anode side of resistor 24'. Thegain of each of amplifiers I0 and I9 is varied automatically byconnecting the grids of the amplifiers, through proper pulsating currentfilter resistors 32, to the AVG connection 23 leading to bias source I5.The positive terminal of source I5 is connected to resistor 24' by anadjustable tap element; 49. In this way the magnitude of the AVG biascan be selected; the source I5 providing the maximum amplification biasfor each amplifier III and I9.

The :diode 25 is connected in reverse manner across circuit 2|.Condenser 4I connects the diode cathode to the terminal 42 of circuit2|, Whereas the diode anode is grounded. The load resistor 25 isconnected between the electrodes of diode 25 and the audio voltagecomponent of rectified current, flowing through resistor 25', isimpressed upon the grid of audio tube 59 by condenser 5|; the latterconnecting the grid of tube 50 to the positive side of resistor 25. Theaudio amplifier tubes 30 and 59 are connected in pushpull relation; themanner of connecting the diodes 24 and 25 to circuit 2I making itpossible to operate the audio amplifier grids in push-pull from thediode load resistors.

The gain of each of tubes I3 and 20 is automatically varied byconnecting the ABC lead 23 (these letters designating the automatic bandwidth control circuit) to a desired point on load resistor 25. The gridsof tubes I3 and 29 are connected, through bias source I6, to theadjustable tap element 60, and the latter can be varied to adjust themagnitude of the band width control bias to be impressed on tubes I3 and20. The source It provides cut-off bias for tubes I3 and 20 in theabsence of received signal energy. When signal energy is rectified bydiode 25, a positive direct current voltage is developed for overcomingthe initial cut-ofi bias due to source I6. The positive bias applied totubes I3 and 20 increases with signal carrier amplitude increase, and,hence, the transconductance of each tube I3 and 20 increases.

The audio output of the push-pull stage 3B-5ll may be utilized in anydesired manner. For example, one, or more, audio amplifiers may follow,and a final reproducer will terminate the receiver system. The anodeside of resistor 24' is preferably connected to control the gain of one,or more, of the signal transmission tubes preceding I. F. amplifier I0.Since the AVG connection to the pre-I. F. stages will furnish effectivegain control, the tap 40 can be set at an intermediate point on resistor24'.

The taps 40 and 60 are related in adjustment in a predetermined manner.In general, the most effective band width control is secured byadjusting tap 60 towards the positive end of resistor 25, and settingtap 40 adjacent the cathode terminal of resistor 24'. This is because ofthe fact that the band width. in the filter depends on the square rootof the product of the positive and negative transconductances of the twotubes in each section; whereas the amplification of each section dependson the square root of the ratio of said transconductances. From this itfollows that moving tap 60 to the upper end of resistor 25, with similarmoving of tap 40 to the lower end of resistor 24, results in an increasein the band widening factor and a decrease in amplification as thesignal amplitude increases. The receiver may be operated without any ABCby moving slider 60 to the grounded end of resistor 25'.

However, if freedom of adjustment is desired to secure other relationsbetween band width and amplification, the taps may be permitted toremain independently adjustable. The diodes 24 and 25 may be a tube ofthe 61-16 type if desired, since the latter comprises a common tubecasing housing the electrodes of two diodes. Further, the condensers 26and II may be adjusted in magnitude to maintain a proper impedance matchbetween the filter and its terminating resistance, as the product of thetransconductances is varied.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the particular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. A wave filter composed of sections each of which consists of a pairof pure reactance networks having positive one-way transconductancetherebetween in one direction, and a negative one-way transconductancetherebetween in the opposite direction, said filter being terminated bysubstantially its iterative impedance.

2. A filter as defined in claim 1, wherein each reactance network isarranged to provide rapid cut-off and high attenuations of frequenciesclose to but outside the transmitted band, and also a substantiallyconstant iterative impedance over the greater portion of the'transmittedband.

WALTER VAN B. ROBERTS.

